1. Field of the Invention
The present invention relates to an ultrasonic cleaning method and an ultrasonic cleaning apparatus for cleaning an article-to-be-cleaned immersed in a cleaning fluid. The “cleaning an article-to-be-cleaned” means the removal of an object-to-be-removed (contaminant) adhering to the surface of the article-to-be-cleaned.
2. Description of Related Art
Conventionally, a technology for cleaning an article-to-be-cleaned by irradiating ultrasonic waves to a cleaning fluid in which the article-to-be-cleaned is immersed (hereinafter, referred to as an “ultrasonic cleaning”) is widely known. In accordance with an ultrasonic cleaning, by imparting vibration energy of ultrasonic waves to a cleaning fluid, actions such as the generation of an impact wave by generating and breaking of bubbles due to cavitation in the cleaning fluid, an acceleration of the molecule of the cleaning fluid, and an acceleration of a physicochemical reaction are exerted. As a result, an object-to-be-removed (contaminant) adhering to the surface of the article-to-be-cleaned is removed from the article-to-be-cleaned. Hereinafter, cleaning actions obtained by using the ultrasonic cleaning may be referred to as “ultrasonic cleaning actions”, and the above-described actions may be referred to as “conventional ultrasonic cleaning actions”.
Meanwhile, a technology for cleaning an article-to-be-cleaned by supplying bubbles (particularly, bubbles with a diameter of 100 μm or less, may be referred to as microbubbles) into a cleaning fluid in which the article-to-be-cleaned is immersed (hereinafter, referred to as a “bubble cleaning”) is also known. In accordance with a bubble cleaning, effects such as an adhesion of oily content to the surface of bubbles, detachment of an object-to-be-removed by impact power upon the crush of bubbles by a physical force are exerted. As a result, the object-to-be-removed is removed from the article-to-be-cleaned. Hereinafter, cleaning actions obtained by using the bubble cleaning may be referred to as a “bubble cleaning action”, and the above-described actions may be referred to as a “conventional bubble cleaning action”.
In order to concurrently utilize such an ultrasonic cleaning action and a bubble cleaning action to effectively clean an article-to-be-cleaned, it can be supposed to irradiate ultrasonic waves to a cleaning fluid in a state wherein bubbles exist in the cleaning fluid in which the article-to-be-cleaned is immersed. However, as shown in FIG. 13(a), for example, when normal ultrasonic waves with a frequency of about 20 kHz and a power of about 25 W/L (watt/liter) is irradiated, a phenomenon where bubbles aggregate at places corresponding to the node(s) or anti-node(s) in the oscillation (waveform) of the ultrasonic waves, and the aggregated bubbles coalesce and rise toward the surface to instantly disappear from the cleaning fluid occurs. In other words, the decreasing rate of the bubbles (bubble density) in the cleaning fluid is extremely high. As a result, it is impossible to concurrently utilize an ultrasonic cleaning action and a bubble cleaning action. Herein, bubbles tend to aggregate at the anti-node(s) when the bubble diameter is smaller than the co-called “resonant bubble diameter” of the irradiated ultrasonic waves, while bubbles tend to aggregate at the node(s) when the bubble diameter is larger than the “resonant bubble diameter”.
Therefore, Patent Document 1 (will be mentioned later) describes a technology where ultrasonic waves stronger (specifically, with a larger amplitude) than normal ultrasonic waves as described above are irradiated when ultrasonic waves are irradiated to a cleaning fluid in a state wherein bubbles exist in the cleaning fluid in which an article-to-be-cleaned is immersed. It describes that, thereby, as shown in FIG. 13(b), the bubble is crushed positively by vibration energy of the strong ultrasonic waves to generate a radical, and, by the action of this radical, an object-to-be-removed (particularly, organic contaminant) is instantly decomposed and removed.
Patent Document 2 (will be mentioned later) also describes that, similarly to Patent Document 1, an object-to-be-removed is removed from an article-to-be-cleaned by positively crushing bubbles. As described above, in accordance with Patent Documents 1 and 2, an article-to-be-cleaned is cleaned by using a new action derived from the positive crushing of bubbles, instead of concurrently utilizing an ultrasonic cleaning action and a bubble cleaning action.
However, when an action based on the positive crushing of bubbles is utilized to clean an article-to-be-cleaned, there were some problems such as the necessity of accurate control of the position at which bubbles are crushed (i.e. the position to which ultrasonic waves are irradiated), the difficulty in cleaning a large article-to-be-cleaned due to a narrow area in which cleaning can be effectively performed, the likelihood of the damage of the surface of an article-to-be-cleaned due to the irradiation of strong ultrasonic waves.
[Patent Document 1] Japanese Patent Application Laid-Open (kokai) No. 2007-253120
[Patent Document 2] Japanese Patent Application Laid-Open (kokai) No. 2008-119642